Composite cores and panels

ABSTRACT

The plurality of pieces of low density cellular material, such as foam plastics, form a core panel having opposite side surfaces and with adjacent pieces having opposing edge surfaces extending between the side surfaces. Sheets of flexible material, such as veils or mats or scrim, are adhesively attached to the side surfaces, and portions of one sheet extend between the opposing adjacent edge surfaces for limiting flexing of the panel. The pieces may be tapered, and portions of the one sheet may project between the edge surfaces either partially or fully to form double wall webs. The webs may have flanges adhesively attached to the other sheet on the opposite side. One sheet may also be stretchable in areas not adhesively attached to the pieces to provide for curving the panel from a planar position maintained by the sheet on the opposite side.

FIELD OF THE INVENTION

This invention relates to composite sandwich panels comprising rigidskin materials separated by and bonded to generally lower density corematerials. More specifically, the invention relates to sandwich panelshaving cores which may be conformed to simple or compound curvaturewhile being placed between flexible skins in a curved mold prior to theskin and core components being adhered together by a hardenable adhesiveresin to form a rigid structure.

BACKGROUND OF THE INVENTION

In many applications, composite sandwich panels are partially orentirely curved, rather than having only planar surfaces. A prominentexample is the blades of wind turbines used to produce electricalenergy. These blades are commonly comprised of sandwich panels, and thecores of those panels are required to conform to specified curvatureswithin the blades. The principal core materials used today in theproduction of wind turbine blades are balsa wood and foam plastics. Inprior art, conformability of these materials is often achieved bycutting or scoring through the thickness of the wood or foam core todivide it into a plurality of strips or blocks of rectangularcross-section. These are adhesively connected, on only one face of thecore, to a continuous flexible sheet material, commonly a scrimcomprising fiberglass rovings.

The flexible scrim serves as articulated joints between each of therigid strips or blocks. When the core panel embodying prior art isplaced into a concave mold, with the non-scrim face of the panel againstthe mold surface, the strips or blocks hinge open relative to one other,forming wedge-shaped spaces within the core panel between opposingsurfaces of the strips or blocks. These spaces fill with resin duringthe molding process, thus increasing the weight and cost of the sandwichpanel. The resin within the spaces is not combined with reinforcingmaterial, such as fiberglass, so its structural contribution to thepanel is severely limited.

In some core panels, the low density cellular material is scored mostly,but not entirely, through the thickness of the core, and the flexiblesheet material is omitted. The cutting or scoring of the balsa wood orfoam plastic is generally done using saw blades, and the score linescomprise saw kerf voids which fill with undesired molding resin evenwhen the core panel is in a planar position in a non-curved mold. Thesaw kerfs between the strips or blocks permit a limited degree ofarticulation when the core panel is placed in a concave mold with thescrim against the mold surface, but the core panel retains wedge-shapedvoids between the strips or blocks which fill with excess resin.

SUMMARY OF THE INVENTION

A core panel constructed in accordance with the present inventionpreferably includes a plurality of strips or blocks of low densitycellular material, generally of rectangular or trapezoidal crosssection. Those faces of the strips or blocks which comprise each of theopposing faces of the core panel are adhesively connected to sheets offlexible material, for example, a veil, mat or scrim which includesfiberglass rovings having sufficient tensile strength to maintainalignment of the strips or blocks during handling and machining.

In the production of composite panels, it is often necessary to machinethe core panels to specified dimensions, for example by cutting withband saws, reciprocating saws or routers. Machining is substantiallyfacilitated if the core panel is rigid enough to prevent it from foldingor otherwise distorting during the machining and associated handlingprocesses. Balsa wood and foam plastics blocks adhesively attached to ascrim material overlying only one face of the core panel have limitedhandling stability, since the rows of strips or blocks are able to foldback upon adjacent rows by a full 180 degrees under gravitational orother forces. Additionally, individual balsa or foam blocks are subjectto being pulled loose or peeled away from the single-side scrim by themachining forces.

The core panels of the present invention provide greatly increased easeof handling and resistance to damage during machining. This featureresults from the fact that all of the strips or blocks comprising thecore panel are adhesively attached to continuous facer mat or scrimoverlying both faces of the core panel. Individual strips or blocks areunable to fold back upon adjacent strips or blocks, since their movementis limited by the veils to which they are adhered. Since the strips orblocks are bonded to and stabilized between two veils, they aresubstantially more resistant to being peeled or pulled away from theveils during machining.

On one face of a core panel constructed in accordance with the presentinvention, edges of opposing strips or blocks are adjacent each other,and the attached first veil maintains this edge alignment duringhandling and machining. The dimensions of this veil substantially matchthe dimensions of the core panel, which are substantially equal to thesum of the dimensions of the faces of the strips or blocks adjacent theveil.

A second veil, adjacent the opposite face of the core panel, is ofgreater width than the sum of the dimensions of the faces of the stripsor blocks which are adhered to it, and portions of this veil extendbetween the edges of opposing strips or blocks. This second veil limitsthe distance by which the edges of opposing strips or blocks may beseparated, thus enhancing the stability of the core panel duringhandling. The veil is sufficiently flexible to fold, or buckle, allowingthe edges of the strips or blocks to which it is attached to be broughtcloser together while conforming the core panel to a mold surface, andthe folded portions of the veil extend between opposing faces of thestrips or blocks. The folded portions of the veil may be lightly bondedto each other, using adhesives which maintain the flatness of the corepanel during machining and handling, but which allow articulation of thestrips or blocks as the core panel is conformed to the mold.

If desired, the folded portions of the second veil, or mat, may be ofsufficient depth to extend through the thickness of the core panel tothe inner face of the first veil. When the mat is comprised of porousstructural material, such as fiberglass, it acts as a structuralreinforcing web within the core panel after infusion with hardenablemolding resin. In this configuration, the core panel may be providedwith a third veil, overlying and adhesively connected to the secondveil. This third veil is of smaller dimensions than the second veil andprovides the stabilizing properties previously described.

If desired, the strips or blocks may be provided with narrow grooves,preferably tapered, parallel to the edges of the strips or blocks,perpendicular to the core panel faces, and extending through most of thethickness of the core panel. Under the pressure of molding in a concavemold, the strips or blocks are forced against the curvature of the mold,which causes the tapered grooves to close, resulting in a reduction ofexcess resin between the surface of the mold and the faces of the stripsor blocks.

Trapezoidal or triangular strips or blocks may be preferred in thoseportions of sandwich panels which have extensive areas of generallysimilar curvature. When conformed to the curved mold, spaces between thestrips or blocks are minimized or even eliminated, reducing resin usage.This embodiment also constitutes an improved method of providing thermalinsulation for cylindrical pipes and tanks, which generally do notemploy hardenable resins.

For composite sandwich panels having shallow radius of curvature orradius of curvature which varies within the panel, it may be desirableto provide the core panels of the present invention with strips orblocks having rectangular cross section. Core panels so constructedprovide enhanced machining and handling robustness, as well as reducedresin usage compared to prior art, as described above.

Other features and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of a reinforced foam corecomposite panel constructed in accordance with the invention;

FIG. 2 is an enlarged fragmentary perspective view of a portion of thepanel shown in FIG. 1;

FIG. 3 is an enlarged fragmentary perspective view of a portion of thepanel shown in FIG. 1;

FIG. 4 is an enlarged fragmentary perspective view of a portion of thepanel shown in FIG. 1;

FIG. 5 is a fragmentary perspective view of a core panel constructed inaccordance with another embodiment of the invention;

FIG. 6 is another fragmentary perspective view of the core panel shownin FIG. 5;

FIG. 7 is an enlarged fragmentary perspective view of another embodimentof a core panel constructed in accordance with the invention;

FIG. 8 is a fragmentary perspective view of the core panel shown in FIG.7;

FIG. 9 is a greatly enlarged section view of a portion of FIG. 7;

FIG. 10 is a greatly enlarged view of a portion of FIG. 8;

FIG. 11 is an enlarged section view of another embodiment of a corepanel constructed in accordance with the invention;

FIG. 12 is an enlarged section view of another embodiment of a corepanel constructed in accordance with the invention;

FIG. 13 is a section view of the core panel shown in FIG. 12, and;

FIG. 14 is an enlarged section view of another embodiment of areinforced foam core composite panel constructed in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, sandwich panel 10 comprises core panel 11 andsandwich panel skins 12. Core panel 11 comprises a plurality of piecesor longitudinal strips 13 adhesively attached to overlying continuousmats or veils 14 and 15 (FIG. 2). Adhesive 16 may be pre-applied toveils 14 and 15 or to strips 13, or it may be provided as a separatesheet material, for example a web-type hot melt adhesive or a hot meltscrim. Strips 13 comprise low density cellular material, for exampleplastics foam or balsa wood. Veil 14 comprises a flexible sheet ofporous fibrous material, for example, glass fiber non-woven veilmanufactured by Johns Manville Company.

The veil 14 is sufficiently flexible to buckle and fold intolongitudinal spaces between foam strips 13 when the veil 14 is subjectedto compressive forces in the plane of the veil and perpendicular to thestrips. Alternately, veil 14 may comprise other adhesively attachedmaterials, for example, nonwoven polyester, open mesh or scrim, parallelrows of fibers transverse to strips 13, or a film which dissolves inmolding resin during the molding process. Veil 15 may be of the samecomposition as veil 14. Alternately it may comprise any of a variety ofother flexible sheet materials, selected for desired structural,cosmetic or other properties. Examples include fiberglass fabric,aluminum, and plywood veneer.

The strips 13, as well as the strips shown in all embodiments of theinvention, may be provided with helically wound reinforcing rovings 17,transverse reinforcing webs 18 or other reinforcing features, forexample, as those described in U.S. Pat. No. 7,393,577, the disclosureof which is herein incorporated by reference. Some or all of thereinforcing materials may be fully or partially impregnated with fullyor partially cured resin, as also described in U.S. Pat. No. 7,393,577.

Strips 13 are of trapezoidal cross section, and the parallel faces ofthe trapezoid strips comprise the faces or side surfaces of the corepanel. If desired, strips 13 may also be provided with longitudinalgrooves 19, which are preferably tapered or V-shaped. If desired, one orboth faces of strips 13 may be provided with surface curvature to matchthe radius of curvature of the mold. In other embodiments of theinvention, strips 13 may have any of a variety of cross-sectionalshapes, for example triangular or circular, or they may comprise hollowtubes, or they may be spaced from other strips.

Core panel 11 illustrated in FIG. 1 is shown as it would appear afterconforming transversely to a curved mold between sandwich panel skins 12and after the application of pressure against the faces of sandwichpanel 10 during the molding process. In this curved position, the edgesurfaces of beveled longitudinal strips 13 are in contact with theopposing edge surfaces of adjacent strips, and tapered grooves 19 areclosed under the applied pressure of the strips against the moldsurface. FIG. 2 shows the planar configuration of core panel 11 prior tobeing placed in the mold. Beveled strips 13 and tapered grooves 19 arein open position, and veils 14 and 15 are adhesively attached toopposing faces of core panel 11.

FIG. 3 shows core panel 11 inverted from its position shown in FIG. 2,as during machining or handling. The arrows shown at bottom of FIG. 4adjacent the right and left sides of the drawing represent supportsalong opposite longitudinal edges of the core panel, as by carrying thepanel to a mold. Flexible veil portions 20 extending between foam strips13 limit the articulation or flexing of the strips, so that panel 11remains flat until inverted for placing in the mold.

FIG. 4 shows the displacement of portions 20 of veil 15 when core panel11 is conformed to curved mold 21. As foam strips 13 begin toarticulate, veil portions 20 which extend between the strips aresubjected to transverse compressive force which causes the veil portionsto buckle along parallel lines between strips 13. Since veil 15 isadhesively attached to the faces of strips 13, it is generallyrestrained from buckling away from the strips and therefore the portions20 buckle inward, folding into the spaces between the beveled edges ofthe strips, in the form shown in FIG. 4 as folded portions 22.

This inward folding feature maintains a smooth surface on the concaveface, so that flexible skin materials applied to the core panel do notwrinkle when in contact with the core panel. However, veils which aresufficiently thin and flexible may buckle outward without compromisingthe sandwich panel skins. FIGS. 9 and 10 show detailed views of veil 15in folded and unfolded positions. Foam strips 13 may be provided withcorner recesses into which the veil portions 22 fold. Referring again toFIG. 4, longitudinal V-groove 19 in foam strip 13 is shown closed inresponse to pressure applied to the faces of the core panel during themolding process. This reduces the volume of spaces 23 between flatstrips 13 and curved mold surface 24, further reducing excess resin.

FIG. 5 illustrates core panel 30 which has been provided with featureswhich permit it to bend both longitudinally and transversely in order toconform to a mold of double or compound curvature. When so conformed,core panel 30 describes a generally part-spherical configuration asshown in FIG. 6. Referring again to FIG. 5, core panel 30 comprises aplurality of low density cellular pieces or blocks 31 adhesivelyattached to flexible veils 14 and 15. All opposite edge surfaces ofblocks 31 are tapered to provide a trapezoidal cross section, aspreviously described in connection with FIGS. 1-4. As core panel 30 isconformed to a mold of compound curvature, those sections of veil 14which overlie the spaces between the pieces or blocks fold into thespaces, as previously described. Blocks 31 are shown as rectangular inplan view, but may be of triangular, hexagonal or other configuration.

Another embodiment of the invention is shown in FIGS. 7 and 8. Referringto FIG. 7, the core panel 40 is shown as it would appear in a flat moldand comprises foam pieces or strips 41 of rectangular cross section andflexible veils or mats 14 and 15, which are adhesively attached toopposing faces of foam strips 41, as previously described in connectionwith FIGS. 1-6. Veil 15 is of greater width than the sum of the widthsof foam strips 41, and when core panel 40 is in planar position, as forhandling or machining, portions 42 (FIG. 8) of veil 15 fold betweenopposing edge surfaces or faces of strips 41 and form folded portions43, shown enlarged in FIGS. 9 and 10.

FIG. 8 illustrates the position of core panel 40 when placed in a curvedmold, with pieces or strips 41 articulated to the open position with thedegree of articulation limited by the width of portions 42 of veil 15.Core panel 40 possesses the same advantages in handling and machining ascore panel 11 described in connection with FIGS. 1-6. Core panel 40 maybe provided with additional handling stability by providing folded veilportions 43, shown in FIG. 9, with weak adhesive attachment between thefolds, which may be overcome while placing the core panel in a curvedmold. Core panel 40 may also be constructed in bi-directional form asgenerally described above in connection with FIGS. 5 and 6.

FIG. 7 also shows an alternate means of providing handling stabilitywhile permitting subsequent articulation of the core panel. Veil 14 maycomprise fibrous material which stretches or yields transversely undertensile force as core panel 40 is articulated with veil 14 adjacent aconcave surface. Portions 44 (FIG. 7) of veil 14 are adhesively attachedto foam strips 41 in spaced longitudinal rows centered on each strip 41,as indicated by dotted lines shown in FIG. 7. Those portions of veil 14not attached to strips 41 may stretch or yield without breaking.

FIGS. 11-14 illustrate embodiments in which portions of the flexible andporous mat or veil which comprises one of the faces of the core panelextend entirely through the thickness of the core panel. Referring toFIG. 11, core panel 50 comprises rectangular foam pieces or strips 41,non-folded veil 51, and folded veil or mat 52 having folds 53 whichextend between opposing side surfaces of strips 41 to the opposite faceof the core panel, terminating adjacent veil 51. Veils 51 and 52 areadhesively attached to opposite faces of strips 41, and portions 53 ofveils 52 may be adhesively attached to opposing surfaces of strips 41for improved handling and machining stability. Strips 41 may rotate orarticulate about the lines of adhesive attachment between veil 51 andfolds 53.

In order to limit the extent of rotation or flexing of core panel 50during handling of the core panel, an additional veil is provided, asillustrated in FIG. 12. Veil 61 is adhesively attached to veil 52 and isfolded between strips 41 when core panel 60 is in planar position. Asshown in FIG. 13, when core panel 60 conforms to mold curvature, strips41 are constrained by veil 61 to limit rotating or articulating freely,thus providing core panel with improved handling properties compared tocore panel 50 shown in FIG. 11.

Referring again to FIG. 11, in a preferred embodiment of the invention,folded veil 52 may comprise a fibrous reinforcing mat of sufficientstrength to impart substantial structural properties to core panel 50.Veil 52 may comprise, for example, a glass fiber nonwoven mat having aweight of one-half ounce per square foot. When folded double as shown inFIG. 11, mat 52 forms a double wall fiberglass web having a weight ofone ounce per square foot between each of strips 41 and extendingbetween opposing faces of core panel 50. When placed in a mold betweenfibrous reinforcing skins such as fiberglass fabric and infused with acurable adhesive resin, double wall folds 53 formed of mat 52 comprisereinforcing members or webs having substantial shear and compressivestrength and also serving to tie the skins of the sandwich paneltogether.

FIG. 14 illustrates another embodiment of the present invention havingstructural double wall webs as described in connection with FIG. 11 andwhich also provides enhanced attachment strength of the structural websto the sandwich panel skin adjacent the termination of the structuralwebs. Sandwich panel 70 comprises reinforced core panel 71 and fibroussandwich panel skins 72 and 73. Core panel 71 comprises foldedreinforcing mat 74 and strips 41 of low density cellular material, suchas plastics foam. During the manufacturing process, folded webs 75,formed from structural mat 74, are extended beyond the face of corepanel 71, and the extended portions of mat 74 are folded onto thesurface of strips 41 to form structural flanges 76 adjacent and parallelto sandwich panel skin 73. Flanges 76 greatly increase the area ofstructural attachment of folded webs 75 to sandwich panel skin 73.

Foam strips 41 may be provided with shallow grooves 77 into whichflanges 76 fold, in order to maintain flatness of the face of core panel70. If desired, core panel 71 may be provided with a surface veil tolimit its articulation, as described above in connection with FIGS. 12and 13, and core panel 71 may be provided with transverse reinforcingmembers as previously described herein. It is within the scope of theinvention to invert one or more of the low density cellular strips whichcomprise a core panel so that the core panel may conform to a moldhaving curvature which reverses from concave to convex.

While the methods and forms of core panels and sandwich panels hereindescribed constitute preferred embodiments of the invention, it is to beunderstood that the invention is not limited to the precise methods andforms described, and that changes may be made therein without departingfrom the scope and spirit of the invention as defined in the appendedclaims.

What is claimed is:
 1. A composite panel comprising: a plurality ofelongated strips of low density cellular material positioned to form acore panel having opposite side surfaces and with adjacent elongatedstrips having opposing edge surfaces extending between the sidesurfaces; a first sheet of flexible material adhesively attached to oneof the side surfaces of the elongated strips; a second sheet of flexiblematerial adhesively attached to the other of the side surfaces withportions of the second sheet extending between the opposing edgesurfaces of adjacent the elongated strips, wherein the portions of thesecond sheet are effective to limit flexing of the first sheet forlimiting curvature of the panel.
 2. The composite panel of claim 1,wherein the portions of the second sheet extending between the opposingedge surfaces extend completely through the thickness of the panel toform double wall webs and wherein adhesive attaches the double wall websto the first sheet.
 3. The composite panel of claim 2, wherein thedouble wall webs include structural flanges extending parallel to thefirst sheet of material, and the structural flanges are adhesivelyattached to the first sheet of material.
 4. The composite panel of claim3, further comprising a third sheet of flexible material attached byadhesive to the second sheet of flexible material and having double wallportions projecting between the double wall webs to limit flexing of thepanel.
 5. The composite panel of claim 2, further comprising a firstsandwich panel skin of rigid material adhesively attached to the firstsheet of material and a second sandwich panel skin of rigid materialadhesively attached to the second sheet of flexible material.
 6. Thecomposite panel of claim 1, wherein the opposing edge surfaces of theelongated strips are tapered to form V-shaped spaces between theadjacent elongated strips and the portions of the second flexible sheetproject into the spaces between the opposing edge surfaces of theelongated strips in response to flexing of the panel.
 7. A compositepanel comprising: a plurality of elongated strips of low densitycellular material arranged to form a core panel having opposite sidesurfaces and with adjacent elongated strips having opposing edgesurfaces extending between the side surfaces; a first sheet of materialadhesively attached to one of the side surfaces of the elongated stripsand a second sheet of flexible material adhesively attached to the otherof the side surfaces; wherein portions of the second sheet extendbetween the opposing edge surfaces and completely through the thicknessof the panel to form double wall webs, wherein the double wall webs havestructural flanges extending parallel to the first sheet of material,and wherein the structural flanges are adhesively attached to the firstsheet of material.
 8. The composite panel of claim 7, further comprisinga first sandwich panel skin of rigid material adhesively attached to thefirst sheet of material and a second sandwich panel skin of rigidmaterial adhesively attached to the second sheet of flexible material.9. A composite panel comprising: a plurality of elongated strips of lowdensity cellular material positioned to form a core panel havingopposite side surfaces and with adjacent elongated strips havingopposing edge surfaces extending between the side surfaces; a firstsheet of flexible material adhesively attached to one of the sidesurfaces of the elongated strips; a second sheet of flexible materialadhesively attached to the other of the side surfaces, wherein thesecond sheet of flexible material comprises stretchable material havingportions unattached to the other of the side surfaces and whichstretches in response to flexing of the panel for fitting the panel to acurved surface.